Digital magnetic recording media usually comprise a substrate in disk or tape form, and a thin layer of a magnetic-pigmented material or magnetic thin film material on the substrate. The magnetic material may exhibit magnetic anisotropy; i.e., it is energetically favorable for the magnetization vector to assume specific orientations relative to the plane of the magnetic layer. One common example is a medium having predominantly in-plane anisotropy, in which the "easy" (low energy) axes of magnetization lie within the plane. Another common example is a medium having perpendicular magnetic anisotropy, in which the easy axis of magnetization is substantially perpendicular to the plane. The remainder of this application assumes recording media having perpendicular anisotropy, but is applicable to media having other combinations of magnetic anisotropy unless specifically noted otherwise.
A small region of the material may have a magnetic moment directed in either of two directions (parallel and antiparallel) to the recording layer surface normal. The small region can represent the storage location of a single recorded digital data bit, and the two antiparallel directions of the magnetization represent binary values one and zero.
To record data, an external magnetic field is applied to the recording material by a magnetic recording head. (For the purposes of this discussion, only the field component applied by the head perpendicular to the recording medium is considered; also, demagnetization fields associated with the medium itself and the energy of magnetic domain walls are ignored.) The applied field component may be either parallel or antiparallel to the recording layer surface normal. If the coercivity of the material is less than the applied magnetic field, the magnetic moment of the material in the area will align with the direction of the field. The magnetic hysteresis of the material ensures that the bit area remains magnetized in that direction after the external field is removed. The magnetic moment of the bit area is determined by passing a magnetic transducer over the bit area, and sensing the fringe magnetic fields caused by the remanent magnetization. Typically, the magnetic transducer is an inductive playback head, but magneto-resistive or other types of transducers may be employed.
An alternative to magnetic recording is magneto-optic (MO) recording. Magneto-optic recording media usually comprise a substrate and several thin film layers deposited on the substrate. One or more of the thin film layers typically are amorphous rare earth-transition metal (RE-TM) alloy(s). The RE-TM alloy has magnetic anisotropy perpendicular to the plane of the thin film. A small region of the RE-TM alloy may have a magnetic moment directed in either of the two directions perpendicular to the plane of the thin film. The region can represent the storage location of a single recorded digital data bit, and the two perpendicular directions represent binary values one and zero. To read the binary value, the bit area is irradiated with a polarized laser beam. A change in the polarization direction of the reflected light caused by a combination of the Kerr and Faraday effects is detected, indicating the direction of the magnetic moment of the bit area.
To record data, an external magnetic field, often called a "bias"" field, is applied to the recording layer; the applied field is well below the ambient temperature coercivity of the recording layer, and so the applied field does not affect the magnetic alignment of the medium at ambient temperature. Concurrently, a focused laser beam pulse locally heats a bit area to a temperature close to or greater than the Curie or compensation temperature of the alloy. The magnetic coercivity of the RE-TM alloy in the heated bit area greatly decreases; following the laser pulse, the heated region cools back to ambient temperature. If the coercivity of the bit area falls below the magnitude of the external bias field at some time during the heating-cooling cycle, the magnetic moment of the alloy in the bit area aligns with the direction of the bias field. Upon return to ambient temperature, the coercivity of the material again becomes much greater than the applied bias field. The direction of the magnetic moment of the bit area will therefore remain the same as the direction of the external bias field direction during the heating-cooling cycle, even if the bias field direction changes at a later time after cooling, or the recording layer is exposed to relatively weak ambient magnetic fields. Thus, a bit is recorded with a particular binary value, corresponding to the bias field direction imposed during heating by the laser.
First generation magneto-optic recording media have somewhat higher areal storage density (the number of bits per unit of area on the physical recording volume (disk, tape, etc.)) than presently available magnetic storage media. Coincidentally, laboratory magnetic media have achieved storage densities similar to those of magneto-optic media. However, the two different types of media achieve this approximately equal areal density with different combinations of linear transition density (the number of magnetic transitions per unit length measured along a single recorded track) and track density (the number of tracks per unit length measured perpendicular to the track direction).
For magnetic recording, linear transition densities on the order of 2-6.times.10.sup.4 transitions per centimeter and track densities on the order of 4-8.times.10.sup.2 tracks per centimeter combine to produce an areal density of about 1-5.times.10.sup.7 transitions per square centimeter. For magneto-optic recording, assuming a laser wavelength of about 830 nanometers, linear transition densities on the order of 6-12.times.10.sup.3 transitions per centimeter and track densities on the order of 4-8.times.10.sup.3 tracks per centimeter combine to produce an areal density of about 3-10.times.10.sup.7 transitions per square centimeter. In magnetic recording, the linear transition density is limited by current head fabrication and head-to-medium spacing considerations, while the track density is limited by both head fabrication and currently achievable head positioning accuracy. In magneto-optic recording, both linear density and track density are limited by the size of the focused laser beam used to record and read data. For systems employing far-field optics (i.e., conventional lenses), the focused beam size is limited by diffraction to approximately the laser radiation wavelength. The minimum distance between transitions along a track is limited to roughly the beam diameter, while the minimum track spacing is typically limited to 1.5 to 2.0 times the beam diameter.
Magnetic recording offers direct overwrite capability, i.e., the ability to write a new bit directly onto the location of an existing bit, regardless of the former bit value, and without a requirement that the former bit be erased before the new bit may be written.
Early generation magneto-optic media did not have direct overwrite capability, but some recent designs do. In particular, a number of designs utilize laser power modulation, complicated medium structures, and additional magnetic fields to achieve direct overwrite capability.
An alternative method employs substantially a conventional M/O medium and some means for rapidly modulating the applied magnetic bias field, typically a magnetic head spaced from one to one thousand micrometers from the recording layer surface. While these techniques provide direct overwrite capability, they all employ conventional M/O readout of the recorded data. Thus, the playback resolution of these systems is subject to the same beam size resolution limitations as conventional optical recording.